Oxidation resistant coatings for columbium and columbium alloys



United States Patent 3,219,477 OXIDATION RESISTANT COATINGS FOR COLUM- BIUM AND COLUMBIUM ALLOYS Joseph Gruhessich, Astoria, and Lawrence Sama, Seaford, N.Y., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Aug. 14, 1961, Ser. No. 132,270 9 Claims. (Cl. 117-114) The present invention relates to protective coatings for columbium and its alloys, and in particular to tin base alloy coatings for protecting columbium and its alloys against oxidation in air at temperatures in the range of 1100 F. to 1800 F.

Previously it was found that columbium base alloys containing 30-40 atomic percent titanium could be protected for 5004000 hours at 1800 F. by hot dipping in aluminum base alloys, such as Al- Cr and Al11 Si. However, alloys with high titanium compositions are undesirable for high temperature applications because of their inferior characteristics at such temperatures.

It was also known that low titanium alloys could be protected for at least 500 hours at 2300 F. by hot dip- Prior to the present invention an alloy 94 w/o Ch- 5 w/ 0 Mo-l w/ 0 Ti, dipped in 71 w/o Al -15 w/o Cr-9 w/o Si-5 w/o Sn, had exhibited excellent oxidation resistance for more than 500 hours at 2300 F. Since this alloy was one of the alloys of particular interest, it was thought to be a relatively simple matter to produce the desired coating for service at the lower temperatures of 1100 F. to 1800 F. A series of tests were accordingly instituted to study the oxidation resistance of the conventional Al-11 w/o Si and Al15 w/o Cr-9 w/o Si-S w/o Sn dip coatings on various columbium alloys in the temperature range of 1100 F. to 1800 F. The samples tested included the Cb5 w/o Mo-1 w/o Ti alloy just mentioned, together with Cb-l W/O Zr and Cb5 w/o Mo2 w/o Ti-2 w/o Zr of particular interest in this investigation.

Contrary to expectations, it was found that no sample exhibited the expected resistance. The Cb5 w/o Mo-l W/o Ti clipped in Al15 w/o Cr-9 w/o Si5 w/o Sn which had withstood over 500 hours at 2300 F. failed after only to 100 hours at 1100, 1400 and 1800 F. The other columbium alloys likewise failed prematurely as shown by typical data summarized in Tables 1 and 2.

Table 1.Al1l w/o Si DIP OXIDATION TEST DATA (HOURS TO FAILURE) Temperatures Base alloy Coating thickness 1100 F. 1400 F. 1800 F.

Cb-S w/o Mo-l w/o Ti 0.002 24 hours, white oxide. 192 White oxide formed immedi- 144 hrs., very thick adherent hrs, surface oxide (white ately. 312l1rs., surface white oxide all over except powder), edges ragged. oxidized, White powder. edges which are eroded.

Edge failure.

Cb-l w/o Zr 0.002 72 hrs, edges eroded, white 72 hrs., samples destroyed-.-" 72 hrs., samples destroyed.

oxide on surface.

(Db-5 w/o Mo-Z wjo Ti-2 w/o Zr 0.002 144 hrs, white powder oxide 48 hrs, white powder oxide on surface.

144 his, very thick adherent white oxide all over except edges which are eroded.

on surface, under surface not bad.

Table 2.Al15 wio Cit-9 W/o Si-5 W/o S11 DIP OXIDATION TEST DATA (HOURS TO FAILURE) Temperatures Base alloy Coating thickness 1100 F. 1400 F. 1800 F.

Ola-5 wlo Mo-l w/o Ti 0.002 144 hours, coating peeling off, 288 hrs, coating peeling off, 288 hrs, edge failures, surface edges eroded. edges eroded. good. Cb-l w/o Zr 0.002 24 hrs, edge failures, surface 24 120 b hrs, edge failures, 23 144 b hrs, edge failures,

OK. surface good. surface good. Cb-5 w/o Mo-2 w/o Ti-2 w/o Zr 0.002 120 hrs, coating peeling back 552 hrs, coating slightly oxi- 216 288 b hrs., edge failures,

from edges. dized, edge failure 1 sample. surface good.

B Hours to failure, first sample. b Hours to failure, second sample.

ping in aluminum and certain aluminum base alloys such as Al-ll Si. These coatings, however, were found not to afford protection at lower temperatures in the range of 1100 F. to 1800 F.

It is the object of this invention to provide a coating for columbium and columbium base alloys that will Withstand oxidation in air for 500 to 1000 hours in the temperature range of 1100 F. to 1800" F.

It is a specific object of this invention to provide a protective coating for the alloys: 99 w/o Cb-l w/o Zr; 94 w/o Cb-S w/o Mo1 w/o Ti and 91 W/o Cb-5 w/o Mo-2 w/o Zr-2 w/o Ti in the temperature range from 1100 F. to 1800 F.

The foregoing objects are achieved by providing a protective coating consisting mainly of aluminum inter metallic compounds on the columbium or columbium alloys. The coating may be applied by contacting the columbium with tin containing minor amounts of aluminum and zinc at an elevated temperature. Advantageously, the coated columbium may then be given a diffusion treatment at a temperature of about 1900 F. to dissolve excess tin.

Results comparable to those in Tables 1 and 2. were obtained when using a dipping bath of pure aluminum and a bath of Al-S W/O Cr.

The foregoing studies demonstrated the failure of aluminum base coating baths to establish the necessary protective coating on columbium and columbium containing minor amounts of alloying elements for service in the desired temperature range. Further work with aluminum base baths was therefore abandoned.

The present invention resides in the discovery that protective coatings can be derived by contacting the columbium or columbium alloy with a bath containing a major percentage of tin, a minor amount of aluminum, and further additives selected from the class consisting of zinc, iron, chromium and titanium. Two baths found to be especially effective are composed respectively of w/o Sn-lO w/o Al-S w/o Zn and 88.5 w/o Sn-10 w/o A1-1 w/o Cr0.5 w/o Ti. Dipping temperatures of 1300 F. to 1950 F. and dipping periods varying from 2 minutes to over one hour may be used. Preferably a sodium borate flux is used to clean the surface of the bath. Test data summerized in the following Table 3, shows successful performance of the coatings at 1100 F. and 1400 F. in air.

TABLE 3.OXIDATION TEST RESULTS OF COLUMBIUM ALLOYS DIPPED IN Sn-lO W/O Al-5 W/O Zn. ALL SAMPLES TESTED IN THE AS-DIPPED CONDITION Time to failure Run Sample (hrs.) at Base alloy No. N 0.

Cb-5 Mo-l Ti 1,000 NF. 1,000 NF. 500 1,000 NF. 1,000 NF- 1,000 NF.

l Cb-5 Mo-2 Ti-2 Zr NF-No failure.

Successful oxidation resistance at 1100 F. and 1400 F. having been achieved as demonstrated by the results tabulated in Table 3, experiments were conducted to determine the oxidation resistance at 1800 F. Both the 85 Sn- Al-5 Zn dip used in Table 3, and additional coating baths contained a major proportion of tin were used. In all cases listed in the Table 4, infra, the samples were prepared by sandblasting with steel grit. Dipping was done at a temperature of about 1780 F. for periods of about 8 to 10 minutes. A sodium borate flux was used with all of the baths.

4 more, and thicknesses up to 0.005" can be obtained. Dipping at 1700 F. to 1900 F. has been found to be especially satisfactory.

Although columbium and its alloys are protected by the dip coatings of this invention, further advantages may be secured by a diffusion treatment to dissolve excess tin from the coating. A diifusion treatment at 1900 F. for one hour has been found to be satisfactory. As a result of this treatment, excess tin disappears with time at temperature.

Experiments have shown that tin is a necessary component of the bath, and that although pure zinc will form a protective intermetallic coating on columbium, a bath composed of aluminum and zinc without tin will not produce as protective a coating in the temperature range of interest.

It is not known why the tin base baths form superior aluminide intermetallics, but it is believed that excess tin contributes to the improved behavior of the coating by acting as a reservoir for repairing defects in the coating in the initial testing stages.

From the foregoing description it is seen that this invention provides oxidation resistant coatings in the temperature range 1100" F. to 1800 F. by dipping columbium and its alloys in a bath of tin containing aluminum and minor amounts of additional elements. By means of these coatings columbium and columbium-rich alloys having superior high temperature strength can be now used in oxidizing environments for prolonged periods.

What is claimed is:

1. The method of protectively coating columbium and columbium base alloys against oxidation in air at temperatures in the range 1100 F. to 1800 F. that comprises cleaning the surface of the columbium, dipping the cleaned columbium for a period of between two minutes to one hour in a molten bath consisting of at least 83 w/o tin, 10 w/o aluminum and the balance selected TABLE 4.TEST RESULTS ON TIN-BASED COATINGS Time (hrs.) to Failure at 1800 F;

Coating alloy Base alloy Sample No.

83" Z iii ii ii 23? 212 SS3 1 r e 10 A11 1 T1 Cb5 Mo-2 Ti-2 Zr (Melt II) s91 SIT after 900 hrs* Cb-5 Mo-Z Tl-2 Zr (Melt I) 1,100. 1,100- SIT after 1,100 hrs. Cb (Melt I) 43 616 792. Ch (Melt II) 308 396 550. 85 Sn, 10 Al, 5 Zn Dip Cb-5 M0-2 Ti-Z Z! (Melt I) 748 800 SIT after 800 hrs. Cb-5 Mo-2 Ti-2 Zr (Melt II) 572 SIT after 900 hrs SIT after 800 hrs. ig Zr (Melt i l ft 6 0 hr it 620 h a er s a er rs 83 10 5 2 Fe p {c 5 14 Z aan 452 *SIT-Stil1 in test.

The unique feature of the coatings is the addition of aluminum, and aluminum-zinc to the tin dipping bath. Resultant coatings consist mainly of aluminum intermetallic compounds which are oxidation resistant to temperatures as high as 2300" F. for long periods of time. Electron probe data of a Cb-S Mo-l Ti sample dipped in Sn-lO w/o Al5 w/o Zn and oxidation tested at 14 F. for over 2000 hours indicated that the phase Cb Al was formed, modified by titanium and molybdenum. The addition of aluminum to the dipping bath has the further important property of a wetting agent. While tin will not wet the columbium, at least at temperatures up to 2000 F., the addition of a few percent aluminum to the bath causes wetting to occur.

We have found that coatings of intermetallics of significant thickness can be obtained at relatively low aluminum, silicon, chromium and zinc concentrations; as low as a total of 5 percent. The coatings can be applied from 1300 F. to 1950 F. for 2 minutes to an hour or from the group consisting of zinc, iron, chromium and titanium, said bath being maintained at a temperature between about 1300 F. and 1900 F. to coat the columbium, and then removing the coated columbium from the said bath.

2. The method of claim 1 wherein the columbium alloy consists of 99 w/o columbium-1 w/o zirconium.

3. The method of claim 1 wherein the columbium alloy consists of 94 w/o columbium-5 w/o molybdenum1 w/o titanium.

4. The method of claim 1 wherein the columbium alloy consists of 91 w/o columbium-5 w/o molybdenum-2 w/o titanium and 2 w/o zirconium.

5. The method of claim 1 wherein the coating bath consists of w/o tin-10 w/o aluminum and 5 w/o zinc.

6. The method of claim 1 wherein the coating bath consists of 88.5 w/o tin-10 w/o aluminum-1 w/o chromium and 0.5 w/o titanium.

7. The method of claim 1 wherein the coating bath consists of 83 w/o tin-10 w/o aluminum-5 w/o zinc and 2 w/o iron.

8. The method of protectively coating the alloy consisting of colurnbinm-S w/o molybdenum-2 w/o titanium-2 w/o zirconium against oxidation in air at temperatures in the range 1100 1 1-1800 F. that consists in sandblasting the alloy with steel grit to prepare the alloy surface for coating, dipping the alloy in a bath consisting of 85 w/o tinl0 W/o aluminum-5 w/o zinc maintained at a temperature of approximately 1780 F. for a period of approximately 8-10 minutes, and then removing the coated alloy.

9. The method of protectively coating the alloy con- 6 sisting of 99 w/o colurnbium-l w/o zirconium against oxidation in air at temperatures in the range 1100 F. to 1800 F. that consists in sandblasting the alloy With steel grit to prepare the surface of the alloy for coating dipping the alloy in a bath consisting of 8S w/o tin-40 W/O aluminum-5 w/o zinc maintained at a temperature of approximately 1780 F. for a period of 8-10 minutes, and then removing the coated alloy from the said bath.

No references cited.

JOSEPH B. SPENCER, Primary Examiner. REUBEN EPSTEIN, Examiner. 

1. THE METHOD OF PROTECTIVELY COATING COLUMBIUM AND COLUMBIUM BASE ALLOYS AGAINST OXIDATION IN AIR AT TEMPERATURES IN THE RANGE 1100*F. TO 1800*F. THAT COMPRISES CLEANING THE SURFACE OF THE COLUMBIUM, DIPPING THE CLEANED COLUMBIUM FOR A PERIOD OF BETWEEN TWO MINUTES TO ONE HOUR IN A MOLTEN BATH CONSISTING OF AT LEAST 83 W/O TIN, 10 W/O ALUMINUM AND THE BALANCE SELECTED FROM THE GROUP CONSISTING OF ZINC, IRON, CHROMIUM AND TITANIUM, SAID BATH BEING MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 1300*F. AND 1900*F. TO COAT THE COLUMBIUM, AND THEN REMOVING THE COATED COLUMBIUM FROM THE SAID BATH. 